In operation of a high-intensity metal halide discharge lamp, visible radiation is emitted by the metallic portion of the metal halide fill at relatively high pressure upon excitation typically caused by passage of current therethrough. One class of high-intensity, metal halide lamps comprises electrodeless lamps which generate an arc discharge by establishing a solenoidal electric field in the high-pressure gaseous lamp fill comprising the combination of one or more metal halides and an inert buffer gas. In particular, the lamp fill, or discharge plasma, is excited by radio frequency (RF) current in an excitation coil surrounding an arc tube which contains the fill. The arc tube and excitation coil assembly acts essentially as a transformer which couples RF energy to the plasma. That is, the excitation coil acts as a primary coil, and the plasma functions as a single-turn secondary. RF current in the excitation coil produces a time-varying magnetic field, in turn creating an electric field in the plasma which closes completely upon itself, i.e., a solenoidal electric field. Current flows as a result of this electric field, thus producing a toroidal arc discharge in the arc tube.
High-intensity, metal halide discharge lamps, such as the aforementioned electrodeless lamps, generally provide good color rendition and high efficacy in accordance with the principles of general purpose illumination. However, the lifetime of such lamps can be limited by the loss of the metallic portion of the metal halide fill during lamp operation and the corresponding buildup of free halogen. In particular, the loss of the metal atoms shortens the useful life of the lamp by reducing the visible light output. Moreover, the loss of the metal atoms leads to the release of free halogen into the arc tube, which may cause arc instability and eventual arc extinction, especially in electrodeless high-intensity, metal halide discharge lamps.
The loss of the metallic portion of the metal halide fill may be attributable to the electric field of the arc discharge which moves metal ions to the arc tube wall. For example, as explained in Electric Discharge Lamps by John F. Waymouth, M.I.T. Press, 1971, pp. 266-277, in a high-intensity discharge lamp containing a sodium iodide fill, sodium iodide is dissociated by the arc discharge into positive sodium ions and negative iodine ions. The positive sodium ions are driven towards the arc tube wall by the electric field of the arc discharge. Sodium ions which do not recombine with iodine ions before reaching the wall may react chemically at the wall, or they may pass through the wall and then react outside the arc tube. (Normally, there is an outer light-transmissive envelope disposed about the arc tube.) These sodium ions may react to form sodium silicate or sodium oxide by reacting with a silica arc tube or with oxygen impurities. As more and more sodium atoms are lost, light output decreases, and there is also a buildup of free iodine within the arc tube that may lead to arc instability and eventual arc extinction. Furthermore, the arc tube surface may degrade as a result of the ion bombardment. Therefore, it is desirable to prevent the loss of the metallic portion of the metal halide lamp fill and the attendant buildup of free halogen, thereby extending the useful life of the lamp.